1. Field of the Invention
The present invention relates to an apparatus and method for measuring interference fringe patterns commonly encountered in optical metrology. More particularly, the invention relates to apparatus for use in conjunction with a standard monochrome closed-circuit television system for the purpose of measuring either an interference pattern produced by an interferometer or a photograph of an interference pattern, i.e., an interferogram.
2. The Prior Art
Interferometric testing has long been used in optical metrology. The advent of the laser has not only made interferometers more convenient to use but has also extended their range of application. Interferometry is used as a tool in the fabrication, final testing, and system alignment, see, for example, C. Zanoni, "Interferometry," The Optical Industry and Systems Directory Encyclopedia, v. 2, pp. E137-E141 (1977).
For most interferometric measurements, the information is contained in either an interference fringe pattern or an interferogram. The quantitative usefulness of an interference pattern is dependent upon having a method of data extraction and reduction. For a preliminary evaluation, positional deviations of the fringes can be obtained using any of a number of simple manual techniques, see, for example, R. Berggren, "Analysis of Interferograms," Optical Spectra, pp. 22-25 (December 1970). An improved manual means for quantifying the irregularities in fringe patterns is disclosed in a copending application of R. L. Gecewicz and G. C. Hunter, Ser. No. 768,342, filed Feb. 14, 1977 which is also assigned to the assignee of this application. From these measurements, the required surface deviations can be calculated. This is adequate for the preliminary correction or evaluation of optical elements if the required accuracy is .lambda./10 to .lambda./20, and if .lambda. is in the visible or infra-red part of the spectrum.
An increasing number of optical elements require correction and evaluation to accuracies of .lambda./100 to .lambda./200. For these cases, hand measurement is very time-consuming and frequently inaccurate. In order to carry out evaluations and corrections to .lambda./100-.lambda./200, or to objectively certify optics to .lambda./10 or better, an interferogram measuring system with correspondingly better accuracy is required. In order to extract the information from the interferogram, it is necessary to measure the two-dimensional coordinates for an array of points located on the center of the fringes.
The measurement of fringe centers on interferograms has been carried out using a variety of techniques. Most of the techniques use mechanical scanning to produce the photoelectric signals whose equality is the signature for the location of a fringe center, see, for example, G. D. Dew, "A Method for the Precise Evaluation of Interferograms," J. Sci. Instr. 41, pp. 160-162 (1964) and J. Dyson, "The Rapid Measurement of Photographic Records of Interference Fringes," Appl. Opt. 2, pp. 487-489 (1963). Fringe scanning techniques are capable of measuring fringe displacements of less than 0.01 fringe.
Another approach is to locate the center of the optical density curve by using a computer-generated fit to the output of a microdensitometer trace across a fringe, see, for example, R. A. Jones and P. L. Kadakia, "An Automated Interferogram Technique," Appl. Opt. 7, pp. 1477-1482 (1968). A microdensitometer is capable of measuring fringe displacement somewhat more accurately than 0.01 fringe.
Another technique used in an instrument manufactured by the assignee of this application is based upon using an oscillating spot of light to measure optical density gradients on an interferogram. The signature for sensing the location of an interference fringe center is the null in the first derivative of the optical density. Using an oscillating spot of light and synchronous demodulation leads to a considerably simpler instrument which achieves improved precision in the location of fringe centers with a minimum of equipment. However, this technique is extremely slow and, therefore, lends itself only to the measurement of interferograms. Furthermore, it is costly and difficult to automate this approach.
In order to measure interferometer interference patterns without introducing errors and complexity, it is desirable to extract all of the fringe center data very rapidly, i.e., in a small fraction, 1/30-1/60, of a second, because of the fluctuations induced in the pattern by mechanical vibrations and atmospheric effects.
An improved means for sensing fringe centers rapidly is disclosed in my copending application, Ser. No. 788,736, filed Apr. 19, 1977.
Sophisticated, expensive interferometers have been designed and built for the high precision, automatic reduction of interference patterns. One such instrument is disclosed in Gallagher, et al. U.S. Pat. No. 3,694,088, issued Sept. 26, 1972. Another sophisticated digital interferometer is discussed in J. H. Bruning, et al., "Digital Wavefront Measuring Interferometer for Testing Optical Surfaces and Lenses," Appl. Opt. 13, pp. 2693-2703 (1974).
In many industrial applications, it is desirable to reduce interferograms quickly in a simple, economic manner. For example, in the manufacture of high precision, high volume optical components, interferograms, and interferometer interference patterns must be measured in large numbers and at high speed with affordable instrumentation. The aforementioned prior art apparatus and techniques are capable of high precision, but they have the disadvantages of being quite expensive and of being too slow for volume production. Therefore, their utility is restricted to rather limited, specialized situations.